1. Field of the Invention
The present invention relates to a conductive adhesive that is used, mainly in soldering an electronic component or the like to an electronic circuit board, and a circuit board and an electronic component module using the same.
2. Related Art
As a method of soldering an electronic component, a reflow soldering method, a flow soldering method and the like are known, and in recent years, a reflow soldering method is employed for surface mounting of an IC chip, a QFP (Quad Flat Package) and the like because it is less likely to generate a bridge between components and between leads and is excellent in productivity. Usually, in the reflow soldering method, a solder paste prepared by mixing powder of a solder alloy and a flux is used, and the solder paste is printed in a predetermined position on an electronic circuit board using a metal mask, followed by heating in a reflow furnace to achieve soldering.
A Sn—Bi system low-melting-point solder is one of the solders of solder pastes that are used in reflow soldering. In this specification, the solder paste means a material composed of (1) solder powder, and (2) a flux composed of organic acid or rosin for removing an oxide film formed on the surface of solder particles or a metal surface on which the solder paste is to be applied, a solvent, and a thicking agent.
However, in the Sn—Bi system solder, there is a problem that it can be used only in some electronic devices because the solder is fragile.
Especially, in a mobile system such as a mobile phone or a digital camera, improvement in joining strength is demanded because cracking occurs or dropout of a soldered component occurs due to the influence on the joined part by an impact or a vibration from outside by falling or the like during use.
For improving the joining strength, as a joining material alternative to solder, a conductive adhesive prepared by mingling solder powder and an epoxy system adhesive with a flux effect as described in Japanese Patent Application Laid-Open No. 2006-199937 (Patent Document 1) is recently developed.
In this specification, the conductive adhesive means a material composed of solder powder, epoxy resin, a curing agent, organic acid, and a thicking agent.
Such a conductive adhesive is a material that provides joining between an electronic circuit board and a component and enhancement simultaneously in such a manner that it is heated by reflow after being printed in a predetermined position in an electronic circuit board by using a metal mask, and after removal of an oxide film of solder by action of organic acid contained in an epoxy system adhesive with a flux effect, joining by a solder and curing of resin occur.
The constitution of the conductive adhesive disclosed in Patent Document 1 is a conductive adhesive prepared by mixing an epoxy system adhesive with a flux effect, and Sn—Bi system solder powder. Here, the epoxy system adhesive with a flux effect disclosed in Patent Document 1 means an epoxy system resin containing a curing agent and organic acid.
In Patent Document 1, Sn42/Bi58(42 weight % Sn-58 weight % Bi) solder powder having a particle size of 25 to 45 μm is used because as for the particle size of the Sn—Bi system solder powder, when the particle diameter is too small, excellent particle junction is not obtained, whereas when the particle size is too large, excellent printing of a land part of a circuit board having a fine pitch is not achieved.
When such a conductive adhesive is used for mounting of a chip component to an electronic circuit board, after printing the conductive adhesive in a predetermined position by a metal mask or the like, and mounting the chip component, the solder powder is melted by heating.
Here, the conductive adhesive releases the adhesive as a result of melting of the solder. The solder wets to the metal of the soldering part, and achieves solder junction, and the adhesive forms a resin film in such a manner that it covers the periphery of the solder. The resin film starts curing at the same time with melting of the solder by its heat, and the curing completes after end of the soldering.
In the conductive adhesive of Patent Document 1, for achieving smooth progression from solder melting to wetting to the metal part (solder cohesiveness) at the time of soldering, namely, for improving the solubility of organic acid in the epoxy resin or the mixture with the curing agent, dibasic acid having an alkyl group in a side chain is used as the organic acid. It is also described that this organic acid uses 2,5-diethyladipic acid, and the number of carbons of the straight chain is 6 or more, and as the alkyl group of the side chain, an alkyl group having 1 to 5 carbon(s) is used.